This invention relates to optical fibers and to a method of manufacturing a low loss optical fiber having a high numerical aperture. More specifically, this invention relates to an optical fiber wherein the core glass material is melted and fined within a tube made of the cladding material.
Optical fibers (or optical waveguides as they are sometimes called) should have minimum transmission losses and, in many applications, the ability to gather or accept light from the widest possible angle. These properties enable the maximum transfer of energy by the fiber from a light source to an output device.
Known optical fibers comprise a glass core enveloped by a glass cladding layer having an index of refraction (n.sub.2) less than the index of refraction of the core material (n.sub.1). Simply stated, the difference between these indices causes light rays entering the fiber within a specified angle to be reflected internally and thereby transmitted through the fiber. The "cone" of light which can be accepted by a fiber is known as the angle of acceptance and the sine of this angle is referred to as the numerical aperture (NA) of the fiber. For any fiber: EQU NA=(n.sub.1.sup.2 -n.sub.2.sup.2).sup.1/2
The higher the numerical aperture, the greater the ability of the fiber to gather light. For numerical aperture equal to one, the angle of acceptance is 90.degree. which means that all of the light striking the face of the fiber will be coupled into it.
Increasing the numerical aperture, while enhancing light gathering efficiency, introduces problems of dispersion because rays entering at grazing angles will travel longer paths than rays which are perpendicular to the face of the fiber. This means that different parts of a light pulse (for example) will traverse the fiber with different traversal times. As a result, the light pulse at the fiber output will be dispersed or boardened and this limits the data carrying capacity of the fiber.
To overcome this problem, the refractive index of the core may be graded so that it increases (e.g. parabolically) from the circumference to the center of the core. This will cause the light to travel a sinusoidal path, with the speed of the light increasing toward the periphery where the index is lower. Hence, light traveling the longer peripheral paths will travel at higher speeds thereby compensating for the added distance and decreasing the dispersion of the input pulse.
The causes for loss in a fiber are material absorption, material scattering, cladding loss and geometry loss. Material absorption losses occur because of transition metal ions and OH groups in the glass that absorb light [e.g. 1 part per million iron will result in a loss of 100 db per km at 800 nm (nanometers)]. Material scattering is due to imperfections in the fiber core, primarily bubbles, microcracks and debris. Cladding loss exists because of imperfections at the core-cladding interface. Geometry loss is due to bends in the fiber and is an inverse function of numerical aperture. A low loss fiber should have losses no greater than 150 db per km.
Typically, the cladding material of an optical fiber is fused silica (SiO.sub.2) of high purity although other glasses such as borosilicates are also used. The core may be a silica glass to which modifiers are added to increase the index of refraction (and the numerical aperture). Suitable modifiers for this purpose include lead oxide (PbO), barium oxide (BaO) and germanium dioxide (GeO.sub.2). These modifiers, in turn, may cause problems of glass stability, and agents such as calcium, zinc, or alumina may be added as stabilizing agents. To facilitate melting of the glass (for drawing), fluxing agents such as the oxides of the alkali metals (potassium, sodium and lithium) may also be added.
The introduction of these various agents or components into the core glass creates problems insofar as contamination and, therefore, material absorption losses are concerned. The various components themselves may be sources of contaminants but, equally important, the procedure by which a multi-component glass is made can add contaminants which cause substantial material absorption losses.